Igneous comes from the Latin word ‘ignis’ which actually means fire. Igneous rocks are a word used for rocks that have formed by the cooling and hardening of molten lava or magma. The atoms and molecules of melted minerals are what make up magma.
These atoms and molecules rearrange themselves into cool mineral grains as the magma cools, forming rocks as the mineral grains grow together. These rocks are therefore made from minerals that help plants grow. That’s cool.
The upper section of the Earth’s crust is made up of around 95% igneous rock. These rocks can form beneath the earth’s surface or at the surface as lava. Those which began their lives below the surface are called intrusive rocks, while those which cooled on the surface are referred to as extrusive rocks.
When a liquid cools to a solid, it is said to have crystallized. The minerals form into bigger and bigger crystals until they smash into other crystals. Once enough mineral crystals smash together so hard that they can’t be taken apart, the minerals have formed into an intrusive igneous rock. A great example of this type of rock is granite. Granite is often used for kitchen surfaces.
Intrusive igneous rock has coarse grained crystals. That means, if you look closely at the rock you can see the mineral crystals. If you look closely at a piece of granite you will see little flecks of white, black, grey and sometimes pink. These flecks of color are crystals. If you can see the crystals that form on igneous rock, you’re looking at an intrusive igneous rock.
The second type of igneous rock, the extrusive igneous rock, is formed when lava cools and forms into solid rock. This cooling is much faster than the slow cooling that forms intrusive igneous rocks. Extrusive igneous rocks have small grained crystals which are so small you can’t see them. There are many different types of extrusive igneous rocks. The type of rock you get depends on what kind of lava the rock was formed from and how fast the rock cooled.
Many mountains are made out of igneous rocks. Also, many mountains with lots of surrounding igneous rock suggest that the mountain could also be a volcano.
The Earth’s moon is made out of igneous rocks too.
Basalt, granite, pumice and obsidian are examples of igneous rocks. There are over 700 different types of igneous rocks. Here are just a few.
Pumice rocks are igneous rocks which were formed when lava cooled quickly above ground. The little pockets of air can be seen in them. This rock is so light, that many pumice rocks will actually float in water. Pumice is actually a kind of glass and not a mixture of minerals. Because this rock is so light, it is used quite often as a decorative landscape stone. Ground to a powder, it is used as an abrasive in polish compounds and in soaps.
Obsidian rocks are igneous rocks that form when lava cools quickly above ground. Obsidian is actually glass and not a mixture of minerals. The edges of this rock are very sharp. Since the Stone Age, people have used obsidian to make cutting tools and tips for their arrows and spears.
Basalt forms when lava cools quickly. Most of the ocean floor and the Hawaiian Islands are made of basalt. It is also found on the Moon, Venus and Mars. Because it is a common rock and very hard, people used basalt for early choppers and for grinding stones to grind grains like millet and barley. Roman engineers paved a lot of Roman roads with basalt, and today engineers still use a lot of ground-up basalt to make asphalt to pave roads.
Granite comes from the Latin word ‘granum’, a grain. This is because granite is made of lots of smaller bits of quartz and feldspar stuck together. Granite comes in different colors, usually pink to grey or sometimes black and is a very hard stone. Because granite is so hard, people sometimes use it for building stone or statues so it will last a long time but it is very hard to cut. In ancient Egypt, sculptors used granite to carve statues of pharaohs. Today it’s often used in kitchen counter tops.
We hope you enjoyed these facts about igneous rocks!